Horse’s Tail shows how important edges are in defining what we see. By lifting the rope tail that divides a large gray rectangle in half, visitors discover that the rectangle is actually two separate squares that both gradually change from dark to light gray. With the rope tail covering the point where the dark edge of one square ends and the light edge of the next square begins, it is almost impossible to see this difference. Graphics beside the exhibit explain that the eye concentrates on the boundary of the rectangle, simply assuming that the rest of the square on each side of the boundary is the same.